The direct synthesis of dimethyl ether from syngas over hybrid catalysts with sulfate-modified γ-alumina as methanol dehydration components

A series of gamma-Al2O3 samples modified with various contents of sulfate (0-15 wt.%) and calcined at different temperatures (350-750 degrees C) were prepared by an impregnation method and physically admixed with CuO-ZnO-Al2O3 methanol synthesis catalyst to form hybrid catalysts. The direct synthesis of dimethyl ether (DME) from syngas was carried out over the prepared hybrid catalysts under pressurized fixed-bed continuous flow conditions. The results revealed that the catalytic activity of SO42-/gamma-Al2O3 for methanol dehydration increased significantly when the content of sulfate increased to 10wt.%, resulting in the increase in both DME selectivity and CO conversion. However, when the content of sulfate of SO42-/gamma-Al2O3 was further increased to 15 wt.%, the activity for methanol dehydration was increased, and the selectivity for DME decreased slightly as reflected in the increased formation of byproducts like hydrocarbons and CO,. On the other hand, when the calcination temperature of SO42-/gamma-Al2O3 increased from 350 degrees C to 550 degrees C, both the CO conversion and the DME selectivity increased gradually, accompanied with the decreased formation of CO2. Nevertheless, a further increase in calcination temperature to 750 degrees C remarkably decreased the catalytic activity of SO42-/gamma-Al2O3 for methanol dehydration, resulting in the significant decline in both DME selectivity and CO conversion. The hybrid catalyst containing the SO42-/gamma-Al2O3 with 10 wt.% sulfate and calcined at 550 degrees C exhibited the highest selectivity and yield for the synthesis of DME. (c) 2006 Published by Elsevier B.V.